232 A THEORY OF BIRDS’ NESTS. 
bird’s existence, will give a clue, and sometimes a very 
complete one, to the reason why it builds its nest of 
certain materials, in a definite situation, and in a more 
or less elaborate manner. 
I now propose to consider the question from a more 
general point of view, and to discuss its application 
to some important problems in the natural history of 
birds. 
Changed Conditions and persistent Habits as influencing 
Nidification. 
Besides the causes above alluded to, there are two 
other factors whose effect in any particular case we can 
only vaguely guess at, but which must have had an 
important influence in determining the existing details 
of nidification. These are—changed conditions of ex- 
istence, whether internal or external, and the influence 
of hereditary or imitative habit; the first inducing al- 
terations in accordance with changes of organic struc- 
ture, of climate, or of the surrounding fauna and 
flora; the other preserving the peculiarities so pro- 
duced, even when changed conditions render them no * 
longer necessary. Many facts have been already given 
which show that birds do adapt their nests to the situa- 
tions in which they place them, and the adoption of 
eaves, chimneys, and boxes, by swallows, wrens, and 
many other birds, shows that they are always ready 
to take advantage of changed conditions. It is pro- 
bable, therefore, that a permanent change of climate 
would cause many birds to modify the form or 
